A standard radiographic intensifying screen comprises, in order, a support, a light-reflecting layer, a phosphor layer and a surface-protective layer. The support may contain light-reflecting material so as to omit the light-reflecting layer.
The light-reflecting layer is arranged to enhance sensitivity of the radiographic intensifying screen. The higher the light reflectance is, the higher the sensitivity is. Therefore, a radiographic intensifying screen of high sensitivity type is prepared using a support which contains a white pigment to give a high light reflectance. In contrast, a radiographic intensifying screen of high sharpness type is prepared using a support which contains light-absorbing carbon black and has no light reflecting layer thereon. The light-reflecting support serves to return back spontaneous emission produced by phosphor particles on the bottom side (i.e., side in the vicinity of the support surface) to the upper surface. Therefore, the use of the light-reflecting support enhances the sensitivity of the intensifying screen, but gives an adverse effect to sharpness of the resulting radiographic image. In contrast, the carbon black-containing support essentially absorbs and does not reflect the spontaneous emission emitted by the phosphor on the bottom side, so that it serves to give a radiographic image with high sharpness but not to enhance the sensitivity. Most of the commercially available radiographic intensifying screens are designed in consideration of balance between the sensitivity and sharpness required for purposes of respective screens.
The replacement of the light-reflecting support with a TiO.sub.2 -containing light-reflecting layer on the support of the radiographic intensifying screen is also well known. Such light-reflecting support is described in the following patent publications: Japanese Patent Provisional Publication (unexamined) Nos. 48-81495, 51-120694, 59-225400, 61-212799, H2-293700, 63-313100, 63-191100, 62-137596, 62-137597, 56-12600, 62-133400, H2-12100, H2-85800, 59-8782, H2-59193, 59-162500, 58-62600, H3-255400, 60-140200, 52-28284, and 62-203098; Japanese Patent Publication (examined) No. H4-37399; U.S. Pat. No. 3,895,157; and British Patent No. 999,780. Details of the disclosures of these patent publications are described below.
Japanese Patent Provisional Publication Nos. 48-81495, 51-120694, 59-3300, 61-212799, 62-137596, 62-203098 and 60-140200, and Japanese Patent Publication No. H4-37399 contain working examples which show a radiographic intensifying screen having a light-reflecting undercoat layer which uses TiO.sub.2 as the light-reflecting material. The phosphors employed in combination with the TiO.sub.2 -containing light-reflecting layer in these radiographic intensifying screen are those which give emissions in the blue light region.
Japanese Patent Provisional Publication No. 59-225400 describes a comparison example which shows a light-reflecting undercoat layer using TiO.sub.2 of anatase structure (particle size: 0.1 to 0.25 .mu.m). The phosphor employed in combination with this light-reflecting layer is BaFBr:Eu.sup.2+ which gives an emission in the violet color region. The publication describes that this light-reflecting layer is not favorably employable.
Japanese Patent Provisional Publication Nos. H-293700, 63-313100, and 63-191100 describe radiographic intensifying screens of which support is a polyethylene terephthalate film containing TiO.sub.2.
Japanese Patent Provisional Publication No. 62-137597 contains a working example of a radiographic intensifying screen in which TiO.sub.2 particles are employed as light reflecting material for the light-reflecting undercoat layer. The reflectance is 88% in the visible light region. The phosphor employed in the intensifying screen is CaWO.sub.4.
Japanese Patent Provisional Publication No. 56-12600 teaches that the provision of the TiO.sub.2 -containing light-reflecting layer is effective to increase the sensitivity of a radiographic intensifying screen. However, there is no teaching on how to balance the sensitivity and sharpness. Moreover, this publication is mainly directed not to a radiographic intensifying screen but to a radiation image conversion panel which uses a stimulable phosphor.
Japanese Patent Provisional Publication Nos. 62-133400, H2-12100, H2-85800, and 59-162500 describe light-reflecting layers using TiO.sub.2. These light-reflecting layers are not for the radiographic intensifying screen but for the radiation image conversion panel which uses a stimulable phosphor.
Japanese Patent Provisional Publication Nos. 58-62600 and H3-255400 describe working examples in which an electroconductive metal oxide-coated layer is utilized. TiO.sub.2 is mentioned as one of the examples of electroconductive metal oxides. The coated amount is so small as less as 1 mg/cm.sup.2, and therefore no high reflection, thus no high sensitivity, is obtained.
Japanese Patent Provisional Publication No. 59-8782 gives a working example of a radiographic intensifying screen which shows the use of a TiO.sub.2 -containing support film and a comparison eagle which shows the provision of a TiO.sub.2 -containing undercoat layer. The TiO.sub.2 contained in the undercoat layer of the examples is not enough in its amount and density, and satisfactory reflectance and sharpness is not accomplished.
Japanese Patent Provisional Publication No. 52-28284 describes a working example for a TiO.sub.2 -containing light-reflecting layer. The phosphor layer employed in combination appears to have a thickness of more than 180 .mu.m (value calculated from the coated phosphor amount).
U.S. Pat. No. 3,895,159 and Japanese Patent Publication No. H2-59193 both contain a working example in which a TiO.sub.2 containing light-reflecting layer is employed in a thick phosphor layer.
British Patent No. 999,780 describes a working example for a radiographic intensifying screen which utilizes a TiO.sub.2 -containing light reflecting layer. The phosphor employed in combination with the light reflecting layer is one which gives an emission in the blue color region. Moreover, the phosphor layer employed in the working example is rather thick.
Therefore, until now, there is not known a radiographic intensifying screen which shows both satisfactorily high sensitivity and satisfactorily high sharpness. Particularly, for a radiographic intensifying screen of high luminant type having a relative sensitivity (details are described hereinafter) of 250 or more which generally shows poor sharpness, enhancement of the sharpness is required.